A car, van, or smaller truck has its turn signals formed flush to the body of the vehicle. The turn signals on these vehicles provide warning to adjacent vehicles of an impending turn or other change of direction. A turn signal formed flush to the body of a large truck, however, will not necessarily provide sufficient warning to adjacent vehicles. For instance, if a vehicle is positioned at the side of a truck's trailer, a turn signal flush to the truck body will not be visible to that vehicle which may be thirty feet behind the signal. Consequently, many operators of large trucks demand turn signals which protrude from the body of the truck.
As a result of this need for protruding turn signals, a number of problems arise. First, the protruding turn signal is susceptible to collision with adjacent objects. Since the turn signal is positioned beneath the driver's line of sight, the driver is unlikely to notice an impending collision between the turn signal and an adjacent object. Consequently, the driver cannot maneuver to avoid the collision. Moreover, unlike a protruding side mirror which is retractable and thus withstands collisions, turn signals shatter upon impact and thus must be replaced at considerable expense.
Another problem associated with protruding turn signals relates to the additional drag created by these bodies. The extending turn signals add to the total frontal area of the truck, which multiplied by the appropriate drag coefficient, provides an indication of the aerodynamic efficiency of the vehicle. Aerodynamic efficiency exercises an important role in determining fuel economy of a vehicle. A final problem associated with protruding turn signals relates to aesthetics. The awkward, extending turn signals are simply unsightly.